1. Field of the Invention
The invention relates to the field of plasmonics. In particular the invention relates to detectors using one-dimensional confined charge in order to detect electromagnetic radiation and other perturbations such as charged particles.
2. Description of the Related Technology
It has long been recognized that reduced dimensionality and finite size play significant roles in the behavior of elementary excitations in solids. The effects of varying degrees of confinement of electrons, holes, excitons, phonons, polaritons, and magnons on the electrostatic, electronic transport, optical, dielectric, magnetic and thermal properties of solids have been areas of sustained, yet intense, investigation for several decades.
Among these elementary excitations in solids are plasmons, which are oscillations in charge density relative to the background (charges) of screened impurities. A plasmon is the particle resulting from the quantization of plasma oscillations, which are density waves of the charge carriers in a conducting medium such as a metal, semiconductor, or plasma. Of both scientific interest and technological application have been surface plasmons in metal nanostructures for resonant detection and identification of individual molecules based on local enhancements (˜104) of electromagnetic fields using, for example, inelastic light scattering. Of crucial importance to several generations of solid-state semiconducting devices is the altered behavior of electrons and holes when electrostatically confined within one or more planes. When carriers exist in sufficiently high concentrations and/or are in sufficiently excited states, their oscillations are quantized as plasmons, obeying Bose-Einstein statistics. Plasmons in a two-dimensional electron state were first reported and observed in liquid He in 1976, and later in inversion layers of Si in 1977 and GaAs in 1979.
Quasi-one-dimensional structures consisting of electrostatically or compositionally confined strips within epitaxial quantum wells (vicinal growth) first appeared and were investigated more than two decades ago. In 1998, controlled growth of single-crystalline Si nanowires using laser-catalyzed vapor-liquid-solid techniques was first accomplished ushering in a new platform for one-dimensional materials and devices. In subsequent work reported primarily by investigators in the group of Charles M. Lieber at Harvard, syntheses of a range of elemental and binary semiconductor nanowire compositions with control of diameter using metal nanocluster catalyst particles were reported. Significantly, the tremendous utility of the nanowire platform was further advanced in 2002 when Gudiksen and Lauhon, et al. demonstrated axial modulation of composition and dopant in the GaAs and GaP family to form superlattices within individual nanowires. Equally significant was the demonstration, soon after, by Lauhon and Gudiksen, et al. of co-axial nanowires consisting of Si cores on which multiple epitaxial shells composed of Ge and Si were grown. In these works, the authors also fabricated and characterized heterojunction diodes, LEDs and FET devices with these new nanowire and device geometries, and measured the optical response thereof.
Devices based on plasmon action may have an impact on sensing comparable to what the discovery of transistor action had on electronics, that is, while the risks of crafting a dense plasma in a solid in nanoscale is high, its pay-off is proportionally high. Detection of the terahertz (THz) range of the electromagnetic spectrum, as well as other ranges of the electromagnetic spectrum, can play an important role in a variety of different technological and commercial fields.
Typical THz components fall into two categories: sources and detectors. Other components such as waveguides, filters, antennas, amplifiers, and THz materials are also important to THz technology. Terahertz sources are difficult components to realize. The reasons include the high frequency roll-off of the electronic solid-state sources due to the reactive parasitics, difficulties that tubes face because of metallic losses and scaling problems, and low level photon energies (meV) of solid-state lasers operating at this range, where the energy is comparable to the relaxation energy of the crystal. As far as power level is concerned, the frequency conversion techniques, either up from a millimeter wave, or down from the optical and IR range, have been the most successful techniques.
One of the components that has received a lot of attention is the diode frequency multiplier. Varactor diode and Schottky diode multiplier circuits have been introduced for multiplying MMW signals up to a few hundred GHz. Recently, a 200 to 2700 GHz multistage frequency multiplier was introduced using Schottky diodes on an extremely thin GaAs substrate and was developed as a source. A sub-millimeter-wave sideband generator consisting of a whisker contacted Schottky varactor mounted in a waveguide was another recently developed source. Some other methods of THz generation as reported by Kolodzey et al. are quantum well intersubband transition in SiGe, boron doped resonant state transition in strained SiGe, and impurity transitions in doped Si.
A unique feature of THz frequencies compared to shorter wavelengths is that the ambient background thermal noise almost always dominates the naturally emitted narrowband signals. Therefore, either cryogenic cooling or long integration time radiometric techniques, or both, are typically required. Currently, there are no existing methods for detecting THz radiation at room temperature. Using the instant invention's nanowire plasma devices as high-speed, room temperature THz detectors can overcome these drawbacks in the prior art. Furthermore, utilization of one dimensional nanowire technology can also serve a role in providing fast and reliable detectors of other ranges of electromagnetic radiation.
Confinement of elementary excitations in one or more dimensions has enabled the development of numerous and important advances in electronic and photonic devices. Controlled variation of the energy level spacing and availability of electronic states using geometric or electrostatic confinement of carriers in quantum wells, quantum wires and quantum dots has led to important advances in transistors, diodes, LEDS, photodetectors and LASERS. Experimental realization of systems in which carriers are effectively confined within planes, along one-dimension, in current rings, or through narrow constrictions or islands, have attracted enormous interest. Detailed investigations in mesoscopic systems have uncovered a range of exciting and unique electronic transport properties, including electron cavities, Kondo physics, the Aharonov-Bohm effect, and other effects, including quantum cascade LASERS, phonon confinement; optical phonon, and exciton confinement.
The current state-of-the-art with respect to high speed transistor technology relies on modulation doping of heterostructures, allowing carriers to be effectively screened from dopant ions, and thus their travel to be subject only to lattice and external forces. These high electron mobility transistors (HEMTS) are characterized by a high-density two dimensional electron (hole) gas (n, p˜1012 cm−3) and a heterojunction that exists between wide and narrow band-gap materials. Doping of the wide band-gap material introduces carriers that are transferred to the narrow band-gap material, and confined due to the band-gap discontinuity. The two-dimensional electron gas (2DEG) or hole gas (2DHG), under appropriate gating, constitute the conduction channels for n-type and p-type HEMT devices. In a HEMT device, gating of this channel of charge produces a transistor action and the speed of the device is limited by the transit time of the carriers from source to drain. The model is that of a reservoir-channel-reservoir with the channel consisting of the 2DEG (or 2DEHG), and the reservoirs of electrons being ohmic metals of the source and the drain. Transit of electrons in this 2DEG (2DHG) is expedited since scattering by ionized dopants is removed; the conduction of the channel remains bounded by the electric-field dependent drift velocity of electrons, or holes. However, a different mechanism of charge, or energy, transport exists when a reservoir of carriers is sandwiched between two barriers: here a carrier density wave, similar to a wave in a pond, propagates and is subject to different constraints. When spatially confined, these electron density waves become quantized forming plasmons. These plasmons can be produced for ultra sensitive detection of external perturbation.
Therefore, there exists a need for applying plasmon-based devices to improve sensing of terahertz radiation and other electromagnetic radiation in order to provide viable, highly sensitive, detectors.